The present invention relates to a radio frequency (RF) resonator and an apparatus using a RF resonator circuit. More specifically, the invention relates to an ion-trapping device namely a quadrupole ion trap.
The quadrupole ion trap was initially described by Paul et al. in U.S. Pat. No. 2,939,952 and normally consists of three electrodes; a ring electrode and two end-cap electrodes, one on each side of the ring electrode. A radio frequency (RF) voltage is normally applied to the ring electrode, and the two end-cap electrodes arc normally grounded. A coil is connected to the ring electrode forming a LC resonator together with the capacitance between the ring electrode and the two end-cap electrodes as well as the capacitance of all other circuit elements connected to the ring electrode. Because of the high Q-value of the resonator, even a low voltage RF driver, which is connected to the coil directly or through transformer coupling, can effectively produce a high RF voltage on the ring electrode.
A quadrupole ion trap can be used as an ion-trapping device of a mass analysis apparatus together with a variety of ion sources. One of the most popular ion sources is liquid chromatography with electrospray ionization. Another ion source, which seems a very promising combination with the ion trap, is matrix-assisted laser desorption/ionization (MALDI).
The ions produced by a MALDI ion source are inevitably pulsed and synchronized to the laser pulse. The efficiency of trapping these ions can be maximized by using a method of fast start of RF voltage as described in PCT Application No. PCT/GB99/00083. Unfortunately, an ordinary RF driver circuit provides only a low excitation voltage, and the rate of increase of RF voltage will then be quite low. The time to establish a required RF voltage usually requires several cycles of RF or more than that.
One method of fast start and/or fast termination of a RF resonator is described in PCT application PCT/GB98/03856. In this method a RF resonator consists of a coil, a capacitor means and two switch means, each having an internal resistance. One end of each switch means is connected to a junction of the coil and the capacitor means where a RF voltage is provided. Another end of each switch means is connected to high voltage power supplies having opposite polarities. The method comprises closing one of the switch means for a short period of time for fast start of the RF resonator, and/or closing both of the switch means for a time interval for fast termination of the RF resonator. Although this method is capable of achieving fast start and/or fast termination of a RF resonator, it may be difficult, in some situations, to sustain high RF voltages. The switch means usually consists of semiconductor switching devices such as MOSFET, IGBT, bipolar transistor, thyristor, etc. These semiconductor devices, while they are not in the conducting state, have capacitance strongly dependent on the applied voltage. In these circumstances, there may arise a shift in resonant frequency with increasing RF voltage, due to a change in the capacitance of the switching device.
A change of capacitance is more significant when the voltage applied on the switching device approaches zero. At fast start of the resonator, one of the switch means is closed and a voltage of the ring electrode approaches the voltage of the high voltage power supply to which the switch means is connected. At that moment a voltage applied on the switch means is close to zero. The resonator may then start to oscillate at a frequency shifted relative to the initially adjusted frequency, and it may then be difficult for the RF driver circuit to sustain this RF voltage.
It is an object of the invention to provide a radio frequency resonator which at least alleviates the afore-mentioned problem.
The invention provides an apparatus having a radio frequency resonator, which has a coil, a capacitor means and at least one switch means being associated with another capacitor means, a recharging means and a high voltage supply means, one end of said switch means being connected to a junction of said coil and said capacitor means where a radio frequency voltage is provided, and another end of said switch means being connected to ground by said another capacitor means and to said high voltage power supply means by said recharging means.
The invention also provides an apparatus having a radio frequency resonator which has at least one switch means, one end of said switch means being connected to a junction where a radio frequency voltage is provided, and said switch means comprising a plurality of switching devices, each said switching device being associated with a parallel capacitor and, optionally, a resistor.
In the MALDI ion source, ions are produced by a laser pulse directed at the sample surface, and are guided into an ion trap using ion optics built inside the MALDI ion source. Before ions are produced, the RF voltage applied on the ring electrode is zero. When the ions are inside the ion-trapping region surrounded by three electrodes, said switch means is closed to charge the capacitance, CRF, between the ring electrode and the end-cap electrodes, the capacitance due to additional circuitry, for example a measuring circuit of the RF voltage, and a parasitic capacitance. This charge is supplied from another capacitor, CBIAS, connected on the other side of the switch means. Once the capacitance CRF is charged, the switch means is opened immediately to initiate a free oscillation of the resonator. The voltage which appears on the ring electrode after the fast start is determined by a ratio of capacitances connected on each side of the switch means, CRF and CBIAS, and becomes lower than the voltage of the high voltage power supply. Then the capacitor CBIAS is recharged to a voltage of the high voltage power supply. Although the voltage applied on the switch is close to zero immediately after the fast start, it increases with a re-charging time constant determined by the capacitor CBIAS and the recharging means connected to the high voltage power supply. This recharging time constant is close to or less than a decay time constant of the resonator and, even within the re-charging time constant, the voltage applied across the switch becomes large enough to reduce a change of capacitance caused by the switch means. Accordingly, a shift of resonance frequency immediately after the fast start of the resonator does not adversely affect the RF oscillation, and the resonator is maintained in a resonance condition.